Process for the densification of a porous structure by boron nitride

ABSTRACT

A process for densifying a porous structure with boron nitride includes placing the porous structure in a borazene precursor of the formula RBNH wherein R is a halogen or hydrogen, heating by induction at a pressure of at least 1.2×10 5  Pa and thereby decomposing the precursor to form boron nitride that is deposited within the pores of the porous structure.

BACKGROUND OF THE INVENTION AND RELATED ART

The present invention relates to a process for the densification of aporous structure by boron nitride e.g. making it possible to densifybidirectional multidirectional felts or fabrics, or porous ceramics. Thedensification consists of filling the gaps of the porous matrix so as tobring about an increase in the density thereof.

The invention also relates to a porous structure densified by boronnitride.

Boron nitride has the following particularly interesting properties:

lightness,

excellent chemical inertia, i.e. resistance to acids and molten meltsand absence of a chemical reaction with carbon or carbon dioxide up to1800° C.,

good thermal stability (up to 2200° C. under an inert atmosphere and upto 2400° C. under nitrogen),

good behaviour during ablation,

good heat conductor,

high electrical resistivity, and

good mechanical properties.

As a result of these specific properties, boron nitride isconventionally used in the construction of vacuum furnaces used at hightemperatures, in the foundry field, in processes for the transformationof very pure metals or alloys in order to bring about a continuouscasting of steels. It is also used in the coating of carbon fibres inorder to protect them against oxidation or serving as an adaptationinterface between the fibres and the ceramic composite matrixes.

Boron nitride is potentially usable for other very importantapplications in the aeronautical or space fields, e.g. in themanufacture of antenna windows, aircraft brakes, thermal shields orreentry vehicles (into the atmosphere) of a dielectric nature. Thus,boron nitride oxidizes much less rapidly than carbon-carbon matrixes andconstitutes a very good electrical insulator. However, for said latterapplications, boron nitride in ceramic form is not suitable, due to itsexcessive brittleness with respect to thermal or mechanical shocks. Toobviate this problem, it is necessary to have materials in the form ofcomposites incorporating a matrix reinforced by woven fibres, as isalready the case for most non-metallic compounds of the carbon, carbide,boride, nitride or oxide type.

Several procedures have been studied in the prior art for producingcomposite materials having a boron nitride matrix.

Thus, a first method is known using the gaseous procedure or vapourphase chemical infiltration. This method has been industrially developedfor carbon and silicon carbide and consists of chemically reacting byheating gaseous species within a porous network known as a preform. Thismethod suffers from numerous disadvantages. It is difficult to obtain ahigh, homogeneous densification, because it is necessary to avoid thepremature closure of the pores located at the periphery of the part tobe densified. In addition, this method is extremely slow and productionperiods of several months are required for large parts.

Among the vapour deposition methods, the article by J. J. GEBHARDT,"Proceedings of the 4th CVD Conference", 1973, pp 460-472 discloses aprocess for densifying fibrous structures of silica and boron nitride byvapour thermal decomposition of a boron nitride precursor. Thisprecursor is e.g. trichloroborazene (BClNH)₃. According to thisdocument, the porous preform is placed within an enclosure into whichthe trichloroborazene is introduced in gaseous form, at a temperature ofapproximately 1100° C. and under a pressure of 4×10² Pa. The articlealso states that attempts to infiltrate boron nitride at higherpressures and temperatures rapidly leads to the sealing of the externalpores of the preform and limits the penetration of the precursor andboron nitride into the porous structure.

The article by Hugues H. PIERSON, "Boron Nitride Composites by ChemicalVapor Deposition", J. Composite Materials, vol. 9, July 1975, pp228-240, describes a composite material formed by chemical vapourdeposition of boron nitride on a substrate based on boron nitridefibres. The reactive gases used are boron trifluoride (BF₃) and ammonia(NH₃). According to this article, a preform formed by a boron nitridefelt is introduced into a CVD enclosure, in the centre of a graphitesusceptor. Induction heating coils are placed in the enclosure walls,said enclosure being supplied by the two aforementioned gases. Thereaction temperature is between 1100° and 1200° C. and the pressuremaintained within the enclosure varies between 40 and 53×10² Pa. In thisway the preform is densified.

However, the materials obtained by the two aforementioned processessuffer from defects and in particular a low density and the inclusion ofnot entirely pyrolyzed compounds. In addition, said composite materialshave cracks.

A second densification procedure consists of carrying out a solid phasereaction. This is described in the article by Ruey Y. LIN, James ECONOMYand H. DEAN BATHA, "Preparation of BN/BN Composites", Ceramic Bulletin,vol. 55, No. 9, 1976. The process described in this article consists ofmixing boron nitride fibres with a partly nitrided precursor constitutedby a mixture of boron nitride and boron oxide B₂ O₃ in fibrous form,followed by hot isostatic pressing or compression.

Finally, French patent application 2 684 366 discloses a process for thepreparation of boron nitride by the reaction of trichloroborazene with aderivative of formula N₂ Si_(n) (CH₃)_(3n) H_(4-n) (n being equal to 2or 3), followed by pyrolysis. However, there is no passage to the vapourphase during pyrolysis.

SUMMARY OF THE INVENTION

The invention aims at obviating the disadvantages of the prior art.

The invention therefore relates to a process for the densification of aporous structure by boron nitride, characterized in that the porousstructure is placed in a precursor chosen from among borazenes offormula RBNH, in which R is a halogen or hydrogen and in that heatingtakes place by induction at a temperature of at least 600° C., under apressure of at least 1.2×10⁵ Pa, so as to form, by decomposition of theprecursor, boron nitride which can be deposited within the pores of saidporous structure.

As a result of these features of the invention, the process makes itpossible to densify porous structures within a very short time of a fewhours, whereas all the prior art processes only permitted densificationafter several days or even several weeks. Moreover, this process makesit possible to obtain a porous structure having a high density andproducts free from cracks. The use of a high pressure avoids thedecomposition of the precursor in the reactor and permanently maintainsprecursor within said reactor.

Preferably, the chosen precursor is trichloroborazene (BClNH)₃.

The invention also relates to a porous structure densified by boronnitride, in which the boron nitride has a graphite-type structure, acrystallite size L_(a) between 50 and 100 nm, L_(c) between 10 and 20 nmand an interplane distance d_(oo2) between 33.3 and 3.37·10⁻¹⁰ m (3.33and 3.37 Å).

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention can be gathered from thefollowing non-limitative description with reference to the attacheddrawings, wherein show:

FIG. 1 Diagrammatically an apparatus permitting the performance of theprocess according to the invention.

FIG. 2 A curve illustrating a thermal densification cycle as a functionof time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the invention, the process consists of placing a porousstructure having a random shape in a boron nitride precursor, heating byinduction under pressure, so as to form boron nitride which can bedeposited within the pores of the porous structure.

This process can be used for densifying substrates of e.g. carbon,graphite, alumina or boron nitride. In more general terms, the porousstructure can be of a random chemical nature, but is stable at theoperating temperature of the process.

The selected boron nitride precursor corresponds to the family ofborazenes of formula (RBNH)₃ in which R is a halogen or hydrogen. Thisprecursor family makes it possible to obtain pure boron nitride.Advantageously, the precursor is trichloroborazene.

Borazenes are compounds which decompose and polymerize rapidly whenbrought to the boiling temperature. Consequently it is necessary tocarry out the reaction under pressure so as to greatly reduce thedecomposition of the precursor. Crystallization of the precursor isavoided by increasing the boiling temperature.

The densification reaction is performed at a temperature of at least600° C. and preferably at least approximately 1000° C., under a pressureof at least 1.0×10⁵ Pa and preferably at least 1.4×10⁵ Pa. The higherthe temperature, the higher the densification speed. Below a pressure of1.2×10⁵ Pa, the precursor very rapidly decomposes, which leads to itsdisappearance and to the stoppage of the densification process after afew minutes.

FIG. 1 illustrates an apparatus usable for performing the processaccording to the invention. This apparatus is in three parts, namely areactor 2, a heat exchanger or condenser 4 and a splash head 6 oraerosol trap. This apparatus is placed within a glove box 8, providedwith inlets and outlets 10 permitting the scavenging of the interior ofthe glove box by a neutral gas current. The glove box 8 ensures thesafety of the operator in the case where the reactor fractures andavoids the ignition of the reaction gases or the inhalation of theproduct by the operator.

The porous structure 12 to be densified is placed on a rotary ornon-rotary support 14, which can also support, in contact with theporous structure, a susceptor 13 (e.g. a graphite mandrel, so as to beheatable by induction). The support is installed in the lower part ofthe body of the reactor 2 and traverses a plug 15 provided with anopening through which can slide said support 14. The support 14 can berotated by a motor 16 located outside the reactor 2.

The reactor 2 is heated by an induction heating device 18 constituted bycoils 20 in which can flow a high frequency current supplied by agenerator 22. The temperature of the porous structure 12 is measured bytemperature probes 24 incorporating thermocouples connected to aprogrammer making it possible to regulate the power of the generator 22in order to control the temperature.

The reactor also has a pipe 26 for the continuous introduction ofprecursor into the reactor 2, a pipe 28 for injecting nitrogen or someother neutral gas for expelling the air contained in the reactor 2 andfinally an orifice 30 provided with a tap located in the lower part ofthe reactor and permitting the emptying of the latter.

The reactor 2 is surmounted by the heat exchanger or condenser 4, whichcomprises a circulation coil 32 for the cooling liquid (generallywater). By cooling the vapours of the precursor and the condensationthereof, the heat exchanger permits their return to the reactor 2.Moreover, the maintaining of an adequate pressure in the apparatusavoids the crystallization of the precursor at said heat exchanger.

The splash head 6 is located in the upper part of the apparatus andserves to eliminate the mist created at the reactor 2 and which wouldnot be condensed at the heat exchanger 4. This splash head comprises afilter 34, a pressure regulating valve 36 and a reaction gas extractionpipe 38. The regulating valve 36 is connected to a pressure gauge 37,which regulates the pressure within the splash head. Finally, the splashhead incorporates a reaction gas extraction pipe 38 permitting thereturn of the gas to a gas treatment installation 40, which is generallylocated outside the glove box 8.

The operating sequence permitting the densification of a poroussubstrate will now be described.

Operating sequence

The porous structure 12 is placed on the support 14 within the reactor 2and then the latter is scavenged with the aid of an inert gas so as toexpel any oxygen which is present in the reactor. The reactor 2 is thenfilled with the precursor 42 in the form of a powder or liquid, as afunction of its nature. This filling takes place by means of the fillingpipe 26. When trichloroborazene is used, the precursor is in powderform.

After putting into operation the cooling circuit 32 and electric powersupplies, i.e. the generator 22, programmer and temperature probes 24,the porous structure temperature is raised. The pressure is fixed at1.4×10⁵ Pa by means of the pressure gauge 37.

As soon as the precursor 42 starts to melt (80° C. fortrichloroborazene), the inert gas scavenging is stopped. The precursoris then in liquid form in contact with the porous structure and in solidform in the lower part of the reactor. The heating power is thenincreased up to the boiling and reflux of the precursor 42 which, inliquid form, penetrates the pores of the porous structure.

On reaching the cracking temperature (e.g. 600° C. fortrichloroborazene), the precursor vapours 42 undergo a cracking in theporous structure 12, which leads to boron nitride deposition within thesubstrate pores.

Cracking specifically takes place at the hottest walls of the porousstructure. When the porous structure is placed on a susceptor, thedensification front is propagated within the cylindrical, porousstructure towards the outer wall. When there is no susceptor or in thecase of a one-piece porous structure (e.g. a plate), the densificationfront advances from the interior of the porous structure towards itsouter walls in contact with the liquid precursor. The densificationfront propagates in the porous substrate during the densificationprocess at a speed which can vary between a few tenths of a millimeterper hour and a few centimeters per hour, as a function of the maximumtemperature of the sample substrate and its nature. The cracking gasespass out through the pores which have not as yet been sealed. The gasesfrom the reaction are discharged in the upper part of the installation.

The substrate is then cooled and then undergoes a heat treatment at atemperature equal to or above 2000° C. for at least one hour, in avacuum furnace under nitrogen (pressure equal to or above 1×10⁴ Pa).This heat treatment makes it possible to stabilize the boron nitriderelative to hydrolysis.

The densified substrates obtained after such a treatment reveal, byoptical micrography in polarized light, the Maltese crosses andcharacteristic columnar growths, by analogy with carbon, with a roughlaminar structure. This structure is the most interesting, because itmakes it possible to obtain by the high temperature heat treatment(2000° C.) a structure of the graphite type.

Electron transmission microscope examinations and X-diffraction analysisconfirm the existence of a good orientation of the crystallites.

The composite materials obtained have a density of approximately 1.8, acrystallite size L_(a) =50 at 100 nm, L_(c) =10 to 20 nm and aninterplane distance d_(oo2) between 3.33 and 3.37·10⁻¹⁰ (3.33 and 3.37Å).

The prior art has not as yet disclosed a boron nitride matrix with saidgraphitization state, A densified substrate production example will nowbe described.

EXAMPLE

The porous substrate 12 to be densified is a parallelepipedic part witha height of 10 cm, a width of 3 cm and a thickness of 1.5 cm constitutedby carbon fibres woven in three directions (fibres T300, 6K, 50 A underregistered trademark TORAY, 50 vol. % fibres). A temperature probe isfixed to the centre of the part. The pressure is fixed at 1.4×10⁵ Pa bymeans of the pressure gauge 37. The reactor 2 used has a height of 200mm and an internal diameter of 5 cm. The precursor is trichloroborazene.

The heat treatment cycle is illustrated in FIG. 2. After a first plateauat 80° C. for 15 minutes, the temperature rise takes place at a rate ofapproximately 400° C./h up to 1000° C. Cooling then takes place at atemperature decrease rate of 800° C./h.

The part obtained undergoes a heat treatment at 2000° C. for 1 hour in avacuum furnace. The final density of the part is approximately 1.8 andthe boron nitride obtained is of the aforementioned graphite-typestructure.

We claim:
 1. A process for densifying a porous structure having pores bydepositing boron nitride in the pores comprising the steps of:placingsaid porous structure in contact with a boron nitride precursorcomprising a borazene having the formula RBNH, wherein R is a halogen orhydrogen, heating said boron nitride precursor at a pressure of at least1.2×10⁵ Pa to a temperature of at least 600° C., and forming boronnitride by decomposing said boron nitride precursor and depositing saidboron nitride in said pores of said porous structure to form a densifiedstructure.
 2. The process according to claim 1, wherein the precursor istrichloroborazene.
 3. The process according to claim 1, wherein theporous structure is of carbon, alumina or boron nitride.
 4. The processaccording to claim 1, wherein the densified structure undergoes a heattreatment at a temperature of at least 2000° C.
 5. The process accordingto claim 1, wherein the porous structure is of graphite.